As solid-state image pickup devices, CMOS (Complementary Metal Oxide Semiconductor) image sensors based on a column-parallel AD conversion method (hereinafter, referred to as a column AD conversion method) in which pixels are two-dimensionally arranged in a matrix and AD converters (ADCs (Analog-Digital Converters) are arranged for individual columns, have been proposed.
In addition, in recent years, CMOS image sensors based on a column AD conversion method that have been improved to be more suitable for high-speed image capturing, have also been proposed. For example, in Patent Document 1, a CMOS image sensor based on a column AD conversion method is proposed in which, by using an up/down counter, a higher frame rate and a higher resolution can be attained without increasing the circuit scale.
Incidentally, recent digital still cameras (image pickup apparatuses) have generally been provided with a static image mode in which static images are captured and a moving image mode in which moving images are captured. In addition, CMOS image sensors based on a column AD conversion method often perform processing while switching the resolution of AD conversion in accordance with the mode in such a manner that a resolution of 12 bits is set for a static image mode, a resolution of 10 bits is set for a moving image mode, and a resolution of 9 bits is set for a high-speed moving image mode.
Now, the resolution of AD conversion includes a “time resolution” on the horizontal axis and a “voltage or current resolution” on the vertical axis in the case that the horizontal axis represents time and the vertical axis represents a voltage value or a current value. In this description, the resolution of AD conversion, or simply resolution, means a “time resolution”. Provided that a CMOS image sensor operates in accordance with a minimum H clock with which the CMOS image sensor can be driven, increasing the resolution of AD conversion means increasing the period of time (the number of counts of H clocks) necessary for AD conversion, and the voltage value or the current value per count, that is, the resolution on the vertical axis, increases in accordance with the AD conversion time period.
Thus, a CMOS image sensor based on a column AD conversion method needs to increase or decrease a comparison time period during which a pixel signal output from a pixel and a reference voltage are compared with each other, in the case of switching the resolution of AD conversion in accordance with a change in the mode. Since a system control device (hereinafter, referred to as an SCU (Signal Control Unit) of the CMOS image sensor controls a pulse timing for controlling the comparison time period, an increase or decrease in the comparison time period can be realized. Specifically, the SCU increases or decreases the comparison time period by changing, in accordance with each mode, a register which determines the length of a pulse for controlling the comparison time period.
Conventionally, as control for changing a register which determines the length of a pulse for controlling the comparison time period, either method 1) in which registers for all the pulse timings are uniformly changed in accordance with the resolution or method 2) in which a register that needs to be changed is selectively changed in accordance with the resolution, has been employed.
For example, a case where the resolution is changed from 10 bits to 12 bits will be explained by way of example. Method 1) for uniformly changing registers is a method for changing registers for all the pulse timings to have a value that is four times the current value at which setting for 10 bits is performed, and has the advantage in that registers do not have to be provided for individual resolutions.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-278135